Project Summary/Abstract Cardiosphere-derived cells (CDCs) exert strong disease-modifying activity in various models of heart failure. In the original funding period of this R01, we tested, and ended up supporting, the hypothesis that the therapeutic benefits of CDCs are mediated by exosomes (nanosized lipid bilayer vesicles that are enriched in noncoding RNAs). Indeed, the concepts first articulated here for heart-derived cells have since been generalized to virtually all other cell types under development as therapeutic candidates for heart failure; even pluripotent stem cell products are now acknowledged to act largely via exosome secretion. We and others have also established that many, if not most, of the effects of exosomes are mediated by their RNA contents, specifically miRs and other noncoding RNAs (ncRNAs). The emerging paradigm is as follows: CDCs secrete exosomes which transfer payloads into target cells, inducing transcriptomic and phenotypic changes that underlie the benefits of CDC therapy. As a corollary of our work to define mechanisms of action of cardiac cell therapy, we have come to realize that exosomes are themselves viable next-generation therapeutic candidates. Potential advantages over the parent cells include product stability, immune tolerability, and the ability to achieve efficacy with simple intravenous (IV) administration. The major focus of the first funding period was on unaltered exosomes produced by primary wild type CDCs. In this competitive renewal, we will manipulate exosomes both by alterations of the parent cell and directly, after isolation. Such manipulation can increase potency as well as improve targeting. Technological and conceptual advances now make it possible to envision a novel, stable, cell-free therapeutic agent that can effectively target heart failure when delivered IV. Here we propose to develop trenchant methods to assess exosome biodistribution, to enhance exosome targeting and potency, and to immortalize CDCs as a stable source of therapeutic exosomes. The specific aims are designed to answer four inter-connected questions: What is the biodistribution of exosomes when delivered IV (Aim 1)? Can we facilitate tissue/cell-directed targeting of exosomes when delivered IV (Aim 2)? Can we immortalize CDCs to generate therapeutically potent and consistent exosomes (Aim 3)? Combining the insights from these various approaches, can we create and select a therapeutic candidate of optimal efficacy for modifying heart failure in vivo after IV administration (Aim 4)? We will create useful models (fate-mapping mTmG mice) and methods (exosomal Cre loading, targeting strategies, CDC immortalization) which will advance our mechanistic understanding of exosome biology. Meanwhile, the proposed work constitutes an important translational step towards the development of exosomes as off-the- shelf therapeutic candidates. CDCs are already in advanced clinical testing, but living cells have limitations relative to cell-free products. Thus, our proposal, focusing on genetically-enhanced exosomes (as cell-free derivatives of immortalized CDCs), opens up new treatment options for heart failure.